Quantifying impacts of vertical land motion and sea level rise on flooding in Lagos Nigeria

Coastal cities face increased flood risk from combined sea-level rise (SLR) and vertical land motion (VLM). Subsidence approximately doubles the rate of sea-level rise experienced along densely populated coasts, yet few exposure assessments resolve the local VLM variability that drives flooding. Here, we integrate high resolution VLM into SLR projections for Lagos, Nigeria. Using GNSS-referenced Sentinel-1 DInSAR (2015–2025), cross-validated against independent PS-InSAR measurements, we find vertical rates from –11.40 to +0.76 mm/yr (P10-P90), the fastest twice the global mean SLR. Combining these rates with tides and IPCC AR6 SSP2-4.5 projections, in a 2 m connectivity-filtered inundation model, we assess three densely populated regions. In the southwest, projected 2100 flood extent increases 70% from 0.98 to 1.68 km². In the central and southeast mainland, where SLR alone produces minimal flooding (0.15–0.16 km²), subsidence expands projected inundation to 1.71–2.28 km². VLM advances flooding timelines by 48-60 years. Propagating elevation, velocity, and sea-level uncertainties, we find sea level projections have the greatest spread, while InSAR uncertainty contributes ≤1.3% by 2100. These estimates are conservative: the surface model masks flooding beneath buildings. This framework is applicable to other subsiding coastal megacities.